The Owen Basin off the south-east margin of Arabia and the evolution of the Owen Fracture Zone

Abstract

Summary. The history of the Owen Basin, which lies between Arabia and the Owen Fracture Zone (OFZ) in the north-west Indian Ocean, is relevant to the break-up of Gondwanaland and the subsequent sea-floor spreading history of the region. Bathymetric, magnetic and gravity studies of the basin were made in 1975 in conjunction with seismic reflection and refraction profiles. A slightly thickened oceanic crust exists under the basin and thickens towards the continental margin. There is limited evidence of an ENE grain in both the magnetic anomalies and the acoustic basement but, on the other hand, there are major ridges which parallel the shelf edge and the OFZ. Two-dimensional gravity modelling indicates areas of abnormally thinned crust under some sediment troughs and areas of abnormally thickened crust under ridges not associated with the OFZ. The Owen Basin does not appear to be a region of simple oceanic crust but possesses features whch possibly relate to the formation of the adjacent continental margin. Free-air gravity anomalies suggest that the OFZ is a continuous feature from 15’ 30” to as far north as, and inclusive of, thg Dalrymple Trough. The arcuate shape of the northern OFZ is probably a relict of an earlier phase of sea-floor spreading before the Gulf of Aden opened although there has been some vertical and horizontal readjustment at the OFZ as a consequence of this opening. Thus the northern OFZ is probably no longer an active transform fault. The Owen and Chain Ridges exhibit rotational symmetry about a point midway between them and the latter ridge, being the inactive extension of a transform fault, traces a history of changing spreading direction during their formation. It is not possible at present to relate the history of the Owen Basin to Mesozoic tectonic events in the Somali. Basin because the age and origin of two intervening seamount chains are unknown.